This thesis investigates the social welfare activities promoted by the lay Buddhist organisation Rissho Koseikai, directed at both members of the organisation and society at large, in order to offer new insight on the public presence of religion in contemporary Japan. In this thesis, I demonstrate how gaps in social care provision institute potential avenues of intervention where religious institutions and individuals can negotiate religion, i.e. mediate religious values and practices and redefine religions position and relevance within Japanese society. Recent scholarship on religiously inspired activism has often interpreted the growing relevance of religious organisations as non-state providers of social services among the signs of a global resurgence of religion in the public sphere. This thesis, however, argues that the social engagement of religious actors, or their contribution to addressing existing deficiencies in social welfare provision, does not necessarily translate into a reaffirmation of the public relevance of religion, or in a redefinition of the position of religious institutions within modern societies. Drawing from ethnographic data collected from local congregations of Rissho Koseikai and surrounding communities, I will illustrate how the movements efforts were frustrated by a number of environmental and structural constraints. These obstacles instituted a need for the negotiation, which took place both inside and outside the religious organisation. Although Koseikai representatives and practitioners eventually managed to address some of the perceived gaps and to an extent fulfilled the religious and organisational goals associated with them, overall their capacity to offer a social contribution was limited. More generally, attempts to negotiate religion through social welfare activities were substantially unsuccessful. In the case of Rissho Koseikai, rather than carving out a space for religion within contemporary Japanese society, efforts to fill the gaps often risked reinforcing its marginality. Data were collected during a 12-month stay in Japan, primarily through participant observation, in-depth interviews and archival research.